Whilst the alphabetic load Index is still in use it does not meant that The PR Ply actually contains for example 8 ply for a LRD tyre. For tyres sold in UK and EU the number of plies and material (IE steel or polyester) is detailed on the sidewall for tread and sidewalls . But you could also see the sidewall embossed "8ply". There are still vehicle that genuinely require 8 ply, or higher tyres but these don't tend to be caravans. this extract is from a Pirelli website on tyre markings.
"Your tyre may also contain a letter indicating the load range. Unlike the load index, this single number does not refer to a specific amount of weight that the tyre can bear. Rather, it more generally specifies the toughness of the tyre and the amount of air pressure that it can hold. It can also be called the ply rating because tyres used to be built with different layers of cords and fabric called plies, and the more plies, the sturdier the tyre and the farther along in the alphabet the ply rating letter was. This type of construction is no longer used, but a tyre with a load range of C, for example, is the equivalent of a 6-ply tyre.
The tyre sidewall may also show information about the materials used to construct the tyre’s sidewall and body. "